net/textproto: properly write terminating sequence if DotWriter is closed with no writes

Fixed textproto.Writer.DotWriter() to properly write \r\n.\r\n to the buffer
when Close() is called without any bytes written. This properly writes the
terminating sequence outlined in RFC 5321 section 4.1.1.4 and RFC 3977
section 3.1.1, even when no other bytes are written.

Change-Id: I262fd2963ee76fff7ffae8e3cb0e86255694b361
Reviewed-on: https://go-review.googlesource.com/c/go/+/77350


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>